Calcium control in crystallization of sodium sesquicarbonate



Feb. 8, 1966 Filed Sept. 26, 1960.

C. BAUER ETAL CALCIUM CONTROL IN CRYSTALLIZATION OF SODIUM SESQUICARBONATE v IIP'Ic J.

FILTER ACTIVATED CARBON AND FlLTER AID |4 t MAKE UP WATER .OCCULATING AG EN T FILTERED TRONA SOLUTION 'SLUDGE TO WASTE CENTRIFUGE 252 2 Sheets-Sheet l CALCINE? soDA ASH THICKENER YSTAL GROWTH MOTER AND C R Y S TALLIZERS TI FOAMING AGENT L/RECYCLE MOTHER LIQUOR DISSOLVE R CR SHED CRUDE TRO MOTHER LIQUOR DIS C AR D Feb. 8, 1966' w..c. BAUER E'I 'AL 1 3 CALCIUM CONTROLIN CRYSTALLIZATION 0F SODIUM SESQUICARBONA'IE 2 Sheets-Sheet z Filed Sept. z; 1960 United States Patent 3,233,983 CALCIUM CGNTRGL IN CRYSTALLIZATIGN 0F SODIUM SESQUICARBONATE William C. Bauer, Green River, Wyo., and Alien P. McCue, Nor-walk, and Kenneth C. Rule, Noroton Heights, Conn., assignors to FMC Corporation, New

York, N.Y., a corporation of Delaware Filed Sept. 26, 1960, Ser. No. 75,455 9 Claims. (Cl. 23-300) This invention relates to improvements in the method of crystallizing certain inorganic salts, particularly those containing sodium and bicarbonate ions, and more particularly sodium sesquicarbonate. This application is a continuation-in-part of our copending application Serial No. 474,828, filed December 13, 1954, now Patent No. 2,954,282.

As is well known, when the solubility of an inorganic salt in a solvent is exceeded, as by sufliciently lowering the temperature of an unsaturated solution or by introducing an excess of one or more of the ions involved, the salt is deposited in solid form. This method is perhaps the most commonly used process for the production of crystalline salts, and is exemplified in the production of sodium sesquicarbonate, where impure trona mineral is mined and purified by recrystallization from a hot, aqueous solution as described, for example, in the Pike US. Patent No. 2,639,217.

In many cases, the type of salt crystals obtained by such methods are of decidedly inferior quality, considering such factors as crystal shape, purity, settling rate, size, uniformity, dewaterability, resistance to breakage and bulk density. The attainment of the desired results as to these characteristics has long been a problem in the production of sesquicarbonate from trona, which prodnot is largely converted to soda ash as the final market commodity. Inferiority of the intermediate sesquicarbonate ordinarily leads to inferiority in the final soda ash product.

It is accordingly an object of the present invention to provide a novel method for obtaining sodium sesquicarbonate of greatly improved quality.

Another object of the present invention is to provide .such a method which is practical and economically feasible.

Another object of the invention is to provide a method of crystallizing sodium sesquicarbonate crystals from aqueous solutions of natural trona substantially free of calcium hardness, whereby improved crystal size and'improved crystal purity is obtained.

Another object of the invention is to provide a method of crystallizing sodium sesquicarbonate from aqueous solutions of natural trona of controlled calcium content, whereby more uniform particle size soda ash may be produced.

Another object of the invention is to provide sodium sesquicarbonate crystals and soda ash made therefrom of improved clarity and freedom from calcium impurities.

Other objects and advantages of the invention will appear from a consideration of the following disclosure and the attached illustrations.

We have discovered that the shortcomings and disadvantages of the prior art methods of crystallizing sodium sesquicarbonate are largely eliminated by the use of very small concentrations of an anionic-active surface active agent and that small variations in the calcium content of the plant liquor from whcih the sodium sesquicarbonate is crystallized will substantially affect the operation of the crystal growth promoting additives. Adopting the name commonly used by chemists, surface active agents are herein called surfactants.

3,233,983 Patented Feb. 8, 1956 "ice The preferred types of these anionic-active surfactants are organic sulfate (organosulfate) or sulfonate (organosulfonate) derivatives, and of these preferred subclasses, the preferred sulfonates are alkyl benzene or alkyl naphthalene sulfonates wherein alkyl carbons total at least four and desirably more, and the preferred sulfates are the higher alkyl alcohol sulfates. Thus, particularly effective compounds are dodecyl benzene sulfonate and polypropylene benzene sulfonates ranging from C alkyl groups; and dibutyl or diisopropyl naphthalene sulfonate.

Another preferred subclass, related to the foregoing preferred subclasses, are the taurates derived from N- alkyltaurine (RNHCH CH SO H) and fatty acids, containing a fatty acid residue of substantial length, i.e., at least 8 carbon atoms. Examples are sodium-N-methyl-N- lauryl taurate, sodium-N-cyclohexyl-N-palrnityl taurate, sodium-N-methyl taurate of tallow acids and sodium N- methyl-N-oleyl taurate. Taurates containing lower molecular weight fatty acid substituent groups are proportionately less effective.

The primary alcohol sulfates containing alkyl groups of substantial size, such as those based on lauryl alcohol, are very effective additives. Examples are sodium, ammonium and triethanolamine lauryl sulfates. Primary alcohol sulfates containing smaller alkyl groups, e.g. on the order of only C alkyl groups, are proportionately less effective and are not recommended. The practical upper limit is about C groups. Examples in the C range include: sodium octyl sulfate, sodium lauryl sulfate, ammonium lauryl sulfate, triethanolamine lauryl sulfate, sodium coconut alcohol sulfate, sodium tridecyl alcohol sulfate, sodium tallow alcohol sulfate, sodium cetyl sulfate and sodium oleyl sulfate.

It appears that in general the free acids of these various surfactants may be used, because they are converted to the sodium salts in the process liquors, which are mildly alkaline, and thus function the same as the soluble salts of the additives.

Cationic-active and nonionic surfactants are totally ineffective as additives in improving the crystallization of sodium sesquicarbonate. Various theories have been considered in an effort to explain the clearly established, unique effectiveness of the anionic-active surfactants particularly the subclasses above enumeratedand the complete lack of effectiveness of the cationic and nonionic classes. In general, these theories have all failed to fully explain this unexpected result.

Up to about parts per million (p.p.m.) of surfactant are recommended in the crystallization of sodium sesquicarbonate, but for various reasons the preferred concentration is about 520 p.p.m. Levels below 5 p.p.m. show beneficial effect, but are not the preferred concentration. Similarly, amounts in excess of 100 p.p.m. may be used with beneficial results, but complicating effects may begin to appear at excessively high levels, such as crystal twinning or branching, and contamination of the product, and the use of these higher concentrations is, of course, economically undesirable. The levels specified herein refer to the active content of the various commercial preparations available, unless otherwise noted. Small variations in the calcium content of the process liquors, however, materially change the effectiveness of the crystal growth promoting additives.

Higher levels of the surfactants were also found in general to be necessary in the sesquicarbonate laboratory testing, to accentuate differences on small scale runs. In contrast, plant scale tests showed great differences resulting from the use of surfactants, where the levels used needed to be only a few percent of those utilized in the laboratory scale runs.

In the process of refining trona to produce pure sodium .tially uniform degree of calciumhardness.

.sesquicarbonate as .described, for example, in said Pike US. Patent 'No. 2,639,217, the surfactant is preferably introduced just ahead of the cooling and crystallization step, so that less is lost in any mother liquor discarded. from the recycling system, or removed by any adsorbent.

used in the purification procedure.

In order to control and provide substantially uniform low calcium content in the process liquors the natural hard Waters of the regionmust be softened'to a substan- Any method of softening the hard inatural waters used in-the process of producing soda ash from natural trona as found- ,in the vicinity of Green River, Wyoming, may be used,

butwe prefer, forreasons of economy, to soften the hard natural waters of this region by thoroughly slurrying'and washing the insoluble trona muds remaining after dissolving most of the sodium values therefrom with thenatural water of; the region, whereby the total hardness of the natural waters, averaging 85120 ppm'as calcium, is reduced to about 16 ppm: and the undissolved sodium values are largely recovered. I

Prior-to-ourdiscovery of the beneficial effects of the use of crystal; growth promoting additives, the trona process of making sodium sesquicarbonate as described, for

example, in said'Pike US. Patent No. 2,639,217 in a commercialplantwas operating at only about. 253()%' of the designed capacity,.due to trouble in the operation of the centrifuges which separate thes'odium sesquicarbonate crystals from the motherliquor.

This difliculty arose from the fact that the crystalline.

slurry obtained in the cooling and crystallization step had veryundesirable properties. Thus, it is desirable that this'slurry be rich in crystal content, and yet the crystal content cannot be so high that the slurry fails to flow properly. Also, the crystals must be of such a'nature that they can be readily separatedfrom the slurry in the centrifuges, i.e. they must settle readily and befieasily However, none of these desirable characdewaterable. teristics was being obtained, and as a consequence ;the

' centrifuges were the bottlenecks inthe plant operations.

Considerable efiort was expended in changing operating methods which, it was believed, might have some effect upon the characteristics of the crystalline slurry or :the

crystals themselves,including various efiorts at improving and varying the purification procedures. However, allof the extensive efforts along these lines failed and the centrifuge bottleneck continued to limit plant production to about one-fourth tonne-third 'plant designed capacity.

Thecrystals continued to be very small andgneedleshaped, and were agglomerated into clumps of feathery appearance; The three points in the process where their undesirable structure was especially troublesome were 'in the settlers, in the concentrating feeders, and in the centrifuges. Due to poor settling the net'result Wasthat a low concentration slurry was being fed into the centrifuges, which in turn delivered a poorly dewatered product at far below designed capacity rates.

Thus, .typical slurries contained on: the order of as little as 35-40 percent crystals by weight, and flowed very sluggishly and unevenly. The use of about 25 'ppm: of one of the preferred additives gave slurries containing 5055 percent crystalsiby weight which, even though containing much more solid material, pumped and flowed smoothly and evenly.

Other undesirable consequences due to the existence.

a break up to furnish additional nuclei.

tals as hereinafter described. 5 p .The invention will be better understoodtrom a consid'-? eration of thefollo'wing examples, and a referenceto the asdescribed, .for example,. in,said ,Pike US. Patent No. 2,639,217 did not initially appear attractive, since it was expected that high concentrations of additives in the process liquors would 'be'necessary at the moment of .crystallization, order to give any substantial benefit.

Difficultyin this respect. was anticipated because the proc-' ess liquor is purifiedwith an adsorbent and filtration just ahead of the crystallization zstep,'and*it wasrealized that this adsorbent would probably effectivelymemove any residual additive carried along. in; the recycledv mother liquor. This meant that the additive would have to be introducedanew man ofthepu'rifiedliquor going to the" crystallizer, which would 'bbviou'slyentail a substantial additional processing cost. It was found that substantially all of the additive remaining: in;the mother liquors recycled to the dissolvers, was removed in theinsoluble muds remaining after dissolution of the-sodium values. from thecrude trona and that any residual additive carried along inthe recycle mother liquor was removed in the. adsorbent filtered from the process liquors just prio 1 to theirintroduction into the: crystallizers.

A review of the published work doneby others 'indi-.

cated that a wide-range. of additives'had beentried .in various. crystallization problems, butjnopattern. existed ,which wouldallow reliable predictions as to the typeofadditive which w'ouldbe efiective. Thus, ionic additivessuch as sodium ,nitrate and sodium. chloride :had been tested .for their ieffect' on thecrystallization of sodium bicarbonate,while at the..other extreme the effectofno'ne ionicadditives had been :noted in the. crystallization of As is well'known, there .is no genother. substances. 7 erally accepted theory on the mechanism of crystallizationrand, as had been noted 'by one of the well-known chemical encyclopedias, the entire =field of crystallize tion is highly empirical Numerous mechanisms have been suggested .for the efifects on aqueous solutions produced by the addition'of asurface active agent. Oneitheory is that a reduction in surface tension around the crystal, 'i.e., at thesolid-liquid interface, allows'easier and more rapid deposition, ere-- ating a bettenbalance between crystal growth-andnucleiation. Another theoryis thatthe additive interfereswith or inhibits growth at the endsofthe crystal, withlthe resulting tendency to produce shorter, thicker crystals ill-L stead of long,slender'crystals which would subsequently r Other workers have theorized that'sur face tension is not a substantial fac:

tor or that, if it is, it is operative only on'cryst'als form-' ing at the air-liquid interface.

V The various impuritiesnaturally present in the process liquors or added during the processing affect theiopera tion of the crystallization improvingadditivesas is shown by the effect. offthe high, calcium'content of the naturalsesquicarbonate crys-' Waters on the size and purity of=the attached illustrations. 7

FIG. :1 is a diagrammatic: flow .dia'gramsimilar to that of the said Pike U.S."Paten't No. 2,639,217 of theprocess used to produce. soda ash from crude Wyoming trona, as modified according to the present invention.

FIG. -2 isa reflection photomicrograph of sodium sesquicarbonate crystals prepared in the laboratory without the use of a surfactant, at .10 power (10X) magnifica-- tion.

FIG..-3 is a refiection-photomicrograph of the rod-like sesquicarbonate crystals prepared with. the use of ppm. of dodecyl benzene sodium sulfate .at the same magnification.

FIG; .4 shows sesquicarbonate crystals prepared on a plant scale without the use of additives, at 10 power (10X) magnification; and

FIG. 5 'shows the .rod-like :sodium sesquicarbonate,

slime.

crystals prepared on a plant scale with the use of ppm. of dodecyl benzene sodium sulfonate at the same magnification.

As illustrated in FIG. 1 the crushed crude trona as removed from the mine is introduced into insulated dissolving tanks 1 in which it is contacted with recycling mother liquor from line 2, which has been reheated to about 100 C. in heater 3. From the dissolvers 1 the trona solution carrying insoluble material therein flows into insulated clarifiers 4 in which most of the mud and slime settles out and is removed through the under-flow line 5 into a head tank 6. The mud and slime discharged from clarifier 4 is about to solids. Makeup water is introduced at 7 and is thoroughly mixed with the muds by pump 8 and pumped into the center of insulated thickener 9 from the bottom of which the sludge is discharged to waste and the clarified overflow, containing trona values dissolved by the makeup water, and the now softened makeup water flows through the line 9a and is added to the recycle mother liquor flowing into the heater 3. The makeup water having an average total hardness of about 120 p.p.m. calculated as calcium, in contact with the trona insolubles in thickener 9, is softened to a hardness of about 16 ppm. in the thickener 9 and the calcium precipitated as calcium carbonate is discharged from the system with the sludge.

In the clarifier 4 a flocculant may be introduced, through the line 10, into the feed from the dissolvers in the center well 11 of the clarifier. The flocculant is preferably introduced in just the right amount to flocculate, settle and remove the colloidally fine slime in the trona solution and be substantially completely removed with the The flocculants which have been found most satisfactory are hydrophylic colloids produced from the seeds of certonia silique, certonia tetragonoloba, cyamopsis tetragonoloba and other leguminous seeds. In general all the cold water soluble gums of high viscosity prepared fiom guar seeds and similar synthetic materials, such as high molecular weight polyacrylamides and hydrolized polyacrylonitriles, are useful as flocculants. One desirable form of such flocculant is sold under the name Burtonite 78 by the Burtonite Company, Nutley 10, New Jersey.

The clarified trona solution overflowing from the weir 4a of the clarifier 4, flows through the line 12 to the filter station 13. Activated carbon and a filter aid is introduced prior to filtration as indicated at 14 and after filtration the hot trona solution flows through the line 15 to the vacuum crystallizers 16, 17 and 18 where the temperature of the solution is reduced to about C. to

crystallize sodium sesquicarbonate therefrom. Just prior to introduction into the crystallizer system a crystallization promoter from the classes described above, and a defoaming agent is added as indicated at 19. The crystallization promoter is preferably used in amounts of 5 to 100 p.p.m. and increases the particle size of the sodium carbonate produced without a crystallization promoter from about 40% plus 100 mesh to above 70% (preferably above 80%) plus 100 mesh when a crystallization promoter is used.

From the last crystallizer the crystal slurry goes to a crystal settler 20, from which a crystal slurry is pumped to a centrifuge station 21 where the crystals of sodium sesquicarbonate are separated from the mother liquor and the mother liquor containing primarily water and dissolved sodium carbonate and sodium bicarbonate together with some calcium and other impurities overflow from the crystal settler 2t and from the centrifuges is recycled through the lines 22 and 23 to the heater 3 where it is reheated and used to dissolve more crude trona. In order to maintain the proper balance of sodium carbonate to sodium bicarbonate in the recirculating mother liquor, a portion of the recirculating mother liquor may be withdrawn and discarded, or processed in other ways to recover the soda values therein, as indicated at 6 24. From the centrifuge station 21 the sodium sesqui carbonate crystals are conveyed to calciners 25 where they are calcined to soda ash.

EXAMPLE 1 Crystallizations without the use of an additive were carried out by cooling sodium sesquicarbonate plant liquor filtrate solutions which were essentially saturated with sesquicarbonate at about C., and containing the usual excess of sodium carbonate. These crystallizations were effected in a 500 ml. flask nearly full of solution, the solution being agitated continuously with a stirring device. The temperature was allowed to drop to 60 C. to promote crystallization and the crystals formed were tested for settling characteristics and dewaterability. It was found desirable to paint the flask on the outside with aluminum paint so that the normal cooling time was about 60 minutes. This method of crystallization satisfled the requirements of simplicity and reproductibility. In numerous runs without the use of an additive, the settied slurry contained about 1220 percent crystals by weight and had a dewaterability, measured by the amount of water retained in the centrifuge cake, of about 8-13 percent. These crystals are shown in Figure 2.

EXAMPLE 2 The procedure described in Example 1 was repeated, but this time an additive was introduced into the solution prior to cooling. The additive used was dodecyl benzene sodium sulfonate, and enough of the commercial material (Detergent D-40 or Sulfonate AA-9) was used to give a concentration of about 150 ppm. The dewaterability was improved to a value of about 3-4 percent and the settled slurry had a crysal content of about 40 percent. When this run was repeated at an additive concentration of 80 ppm., the dewaterabiilty was about 6 percent and the settled slurry contained about 35 percent crystals by weight. See Figure 3 for illustration, and note the striking improvement in comparison with Figure 2.

EXAMPLE 3 Another run similar to the foregoing example, but using butyl naphthalene sodium sulfonate (Sorbit AC, containing about 65 percent active material) likewise gave greatly improved results over Example 1, giving a dewaterability of about 4 percent and a settled slurry containing about 30 percent crystals by weight, when 330 ppm. of the commercial additive was used.

EXAMPLE 4 Another run, similar to the foregoing, was made using 400 ppm. of commercial sodium lauryl sulfate and this time a dewaterability of 3.6 percent and a settled slurry of 39 percent crystal content was obtained.

EXAMPLE 5 The laboratory use of about 380 ppm. of commercial triethanolamine lauryl sulfate gave a dewaterability of 3.1 percent and a 35 percent crystal content settled slurry.

EXAMPLE '6 The laboratory use of 375 ppm. of sodium-N-cyclohexyl-N-palmityl taurate (Igpeon CN-42 containing 28 percent active material) gave crystals with a dewaterability of 3.6 percent and a settled slurry of 39 percent crystal content.

EXAMPLE 7 The use of ppm. of sodium-'N-met hyl-N-oleyltaurate (Igepon T-77, containing 72 percent active material) gave a dewaterability of about 4 percent and a settled slurry of about 37 percent crystal content, in small scale trials.

EXAMPLE 8 The troubles encountered in the operation of the plant process for the production of crystalline sesquicarbonate as illustrated diagramatically in Figure 1 have been described above. Without the use of an additive, the plant centrifuge cake had a moisture content of about 10-15 percent, with the centrifuge turning out about 210 tons per day (TPD) of sesquicarbonate. Thiscentrifugeproduction was only about one-thirdof the designed capacity, and the product had a higher moisture content than contemplated in the designcalculations. SeeFigure 4 for crystal form.

EXAMPLE: 9 The-plant operation was modified by the introduction of dodecylrbenz ent sodium sulfonatejust ahead of the. cooling and crystallization step, at a concentration in the After 1 plant liquors of about SOppm. (active material). operations had reached equilibrium conditions, the centrifuge cake moisture dropped. to about .7. percent,-with the centrifuge production increased to abouti4-20 -TPD. Further' runs showed that production could be raised to 850 TPD, substantially exceeding the designed capacityof 650 TPD of sesquicarbonate.

EXAMPLE 10 Operation of thejplant in amanner similar to that described in Example 9, but at an "additive levelof only 20 ppm/(active-material),*resulted in centrifuge cake rnoistures of about 6 percent, and-a centrifugeproduction rate of about 500 TPD. Further runs showed that even 'at this .additive level and moisture content a production rate of SSO TPD can be maintained.

EXAMPLE 11 A run similar to" Example 10 was made at a level of 10 ppm. surfactant, giving excellent rod-like crystals,

which as shown in Figure 5, have a length several times thegreatest' width.

EXAMPLE 12 Operation of the plant in a manner similar to that described in the two preceding examples, but at an additive :level of only abonfS ppm. (active material), gave centrifuge cake moistures of about 9 percent and centrifuge capacities'of about SSOTPD.

EXAMPLE '13 Using the. surfactant ofExample .6 in the plant atan active material concentration of 14 ppm. gave results very similar to those obtained in Example 11.

Nonionic, cationic-active, dyes and other types of additives were ineffective with sodium sesquicarbonate. It was found that inc reasing .the scale of operations as in plant practice"perm'itted the use of considerably lower I concentrations of additive. as compared with the concen-. trations necessary in the laboratory, without substantially affecting the numerous advantages .realized from the practice-of this invention, and-in most cases actually. giving superior results.

Plant operation, however, continued to be erratic. The

plant would operate for several days or months producing crystals of soda' ash, 80to 90% of which had aparticle' size-of +1 mesh'and thenthe percentage-of +1 00 a mesh particleswould drop to 60-to 70 percentand gradually climb back to 80 to 90% +100 mesh. We believe that this erratic. operationwas caused at times by variationsin the calcium content of the process liquors at the point .of crystallization as is shown by Table? XI herein.

In the" process of dissolving raw crude trona and producing soda ash therefrom as described in said Pike US. Patent No. 2,639,217, and as described in connection with FIG. 1, calcium is introducedinto the process liquors at twopoints. The :insoluble fraction of the crudetrona fed-to -thedissolvers contains appreciable quantities of such calcareous minerals as limestone and shortite -(Na CO '2CaCO and the natural waters of theregion, 7

used as process and makeup water, have an average total hardness of about 270 ppm. calculated as calcium carbonate (of this amount, abouti100 ppm. is'present as .magnesiurnxcarbonate, with the remainder calcium carbonate).

The hardness of the water varies some with the season, beinglowest inthespring, when thawing and -about .230-ppm., the; calcium dissolved from thetrona is substantially uniformfrom day to day. If no attempt is spring rains may'reduce the total hardness temporarily to rnade to reduce the calcium content of theprocess liquors and the hard water natural tothe region is; introduced directly into the: process liquors without priorsoftening the calcium content of the. soda ash will be from 250 to 375 ppm. which isfar above that desired, and we. also havefound that the production of large sizecrystals of sodium sesquicarbonateaccording to this invention is seriously interfered with.

Whereas in plant operation it was lpossible to -produce soda-ash having over mesh-particles andless than 1'O0-ppm. of calcium therein, unexpectedvariations from this performance frequently occurred If the calcium contentof the process liquors wasbelow about30 ppnr, the performance was good,- but ifthis limit was. materially exceeded the particle size of the soda'ash would i drop oft", a more dusty ash wasv produced, i the clear appearance of the se squicarbonate crystals Waschanged to an opaque appearance and the.calciurnI content of. the;

soda ash increased many fold.

We :havefoundthat these difficulties can besubstam V tially overcome by maintaining .thecalcium -contcnt of the process. liquors below specified :limits byintroducing; into the dissolving and crystallizing circuits only softened naakeup water containing preferably less thanZO ppm. of Ca, so that when mixed with the. recirculated mother liquor the calcium content ofithe processliquors going intothe crystallizers is keptbel'ow about '30'ppm., pref-' erably below 25 ppm.. The rangeof Ca in they softened water. should in allleventsv :be bclow 30 ppm. and the- .process liquors should .haveless than 40 ppm. Ca therein at the crystallization step.

The natural hard water of theregion may be'softened 1 by any water softening process, such as the cold-lime soda.

process or the .zeolite process, but We prefer to soften all the'makeup water by thoroughly mixing the makeup water with concentrated slurry of the insoluble trona muds containingsufiicient'undissolved alkali. value,. to

exchange sodiumfor most of the calcium inthewater, and sufiicient concentration ofinsolubles (mud) to flocculate and entrain most of the, calcium carbonate'formed i by the ion exchange and carry it out of, the. process with the insoluble mudsdischarged to Waste.

In order tosoftcnithe' makeup water in thismanner extending from the bottom of the thickener and the clear overflow liquor, reduced incalcium' hardness. toabout 16 ppm; flows through-the line 9a to the recycle mother liquor line 23 where it is mixed-"with the recycle'mother liquor in the ratio of about 1 ipart of softened makeup water-to 18 parts of mother liquor.

The mother liquor returning from the, settler 20' and centrifuges 21 contains about 24 ppm. ofcalciumhardness, and the calciunrhardness'of the makeup water (16 'ppn1.;Ca) added to the mother liquor shoirldbesuch that the total calcium hardness of the combined stream flowing into the *dissolversl is about 23-"ppr n.

In the dissolversl calcium-is dissolvedbut this'amount The, ratioof calcium in the dissolverfeed to the amount of calciumdissolved 5 from the tronashould be such that the calcium content of appears to be somewhat constant.

the feed to 'clarifier -4 is in the range of 20 to' 40 ppm., preferably lessthan 30 ppm. The clarifier overflow, constituting the feed to the filters 13 and consequently to the crystallizers 16,17 and ls should contain preferably not 9 more than 3 ppm. of calcium and preferably of the order of 20 to 25 ppm. calcium in order not to interfere with the operation of the crystallization additives as de- Thickener overflow 16 Dissolver feed liquor (mother liquor combined with makeup water)' i 23 Di-ssolve'r overflow 30 Crystallizer'feed liquor 28 24 Crystal'settler overflow and centrifuge filtrate Light soda ash 100 These limits can, of course, be varied some and will vary some inoperation, but represent desirable limits of calcium content which can bemaintaine'd by softening the makeup water to around 10 to 20 ppm. calcium hardness and controlling the ratio of makeup water to mother liquor to maintain the calcium balance substantially as indicated.

In order to illustrate the effect of the calcium content of thecrystallizer feed liquors on the effectiveness of the crystallization additives described above a plant filtrate (crystallizer feed liquor) reduced in calcium contentto about 11 p.p.m. was crystallized with '100'p.p.'m.of"the same crystallization additive, dodecyl'ben'zen'e sodium sulfonate, as used in Examples 9, 10, 11 and 12, andsi'milar crystallizations were made with increasing concentrations of calcium in the crystallizer feed. The resu'ltswere' as follows: 1

Table II Sesquicarbonate Crystals Ca Content of Soda +60 +100 Ash, mesh, mesh, p.p.m.

percent percent (1) Plant crystallizer feed liquor containing 11 p.p.m. 0a-. 40. 7 73. 0 157 (2) Plant crystallizer feed liquor containing 29 p.p.m. Ca 48. 9 77. 221 (3) Plantcrystallizer feed liquor-containing 34 p.p.m. Ca 14. 7 I 35.0 293 (4) Plant crystallizer feed liquor con- I taining 39 p.p.m. Ca 38. 6 64. 2 389 (5) Plant crystallizer feed containing 54 p.p.mrCanu' 32. 9 56. 7 800 (6) Plant crystallizer feed liquor containing 61 ppm. Ca 24. 3 47. 8 829 (7) Plant crystallizer feed liquor containing 79p.p.m. Ca 33.5 54.9 984 (8) Plant crystallizer feed liquor c0ntaining 111 p.p.m. Ca 30. 3 53. 2 1,080 (9) Plant crystallizerieed liquor containing 211p.p.m. Qa 35.0 54. 6 1, 800

As is shown by the above table, at about 40 p.p.m. calcium in the crystallizer feed liquor, the particle size of the sodium sesquicarbonate crystals crystallized in the preserice of an anionic active surfactant crystal modifier drops rapidly from in excess of 70% +100 mesh to below 50% +100 meshwhen 61 p.p.m. Ca is present in the solution, and the calcium contamination of the soda ash' rises rapidly. The accumulation of calcium scale in various parts of the plant also increases with increase of calcium in the process liquors. The particle size of the soda ash produced from the sesquicarbonate crystals parallels the particle size of the'sesquicarbonate crystals. r

In plant practice even better results are obtained, as more than 80% +100 mesh particles of soda ash containirig less flian 100 p.p.m. of calcium can be regularly pro- 10 duced with a lesser amount of the surfactant when the calcium content of the crystallizer feed liquor is kept below about 30 p.p.m.

In a similar test a synthetic solution of sodium sesquicarbonate similar in composition to the plant crystallizer feed liquor was prepared from reagent grade materials using distilled water and dissolving sodium carbonate and sodium bicarbonate therein. The synthetic solution contained 16 p.p.m. of calcium, due to the calcium content of the sodium carbonate and sodium bicarbonate dissolved therein. Dodecyl benzene sodium sulfate in the amount of p.p.m. was added to this solution. Different amounts of calcium were added and crystallizations of sodium sesquicarbonate from this solution were carried The results of these tests substantially parallel the results of Table II and show that at some point between 16 p.p.m. Ca and 66 p.p.m. Ca the particle size of the sodium sesquicarbonate crystals, crystallized in the presence of an anionic-active surfactant crystal modifier, drops rapidly from in excess of 75% +100 mesh to substantially 50% +100 mesh particles, and the calcium contamination of the soda ash rises rapidly.

In a plant using 200 gallons per minute of makeup water, without softening, approximately 400 pounds per day of calcium carbonate will be introduced into the system every day, which will influence the crystal size detrimentally as shown in Table II and most of which will appear as calcium contamination in the final product and calcium scale in the process lines, tanks, etc. By reducing the calcium content of the makeup water to about one-fourth of the calciurn content of the natural watersby treating it with the thickened trona muds, and maintaining the calcium content in the crystallizer feed liquid below 40 p.p.m., preferably 30 p.p.m., the detrimental effect on the final product is largely overcome and'the sodium values in the trona muds are recovered. While it would be preferable to soften the makeup water to zero hardness by the use of z'eolitic or resin type water softeners, these processes would be more expensive and would not recover the sodium values in the trona muds. While we have referred to the reduction of the calcium content of the natural waters as softening, the object of our process is to control the calcium content of our process solutions and the products produced therefrom rather than to merely soften the natural waters.

It "should be understood that the foregoing descriptive and illustrative materials represent only typical embodiments of the invention, and are not to be construed as limiting the scope thereoflas defined by the claims which follow.

We claim:

1. A process for preparing crystals of sodium sesquicarbonate from crude Green River, Wyoming trona, which crystals are improved in size, clarity, dewaterability and settling rate, which comprises softening the natural hard water of the region, containing an average total hardness of about 85 to ppm. calculated as calcium to a 1 1 calcium hardness of less than 30 ppm. to providemake-up water for said process, adding said make-up watersoftened to a calcium hardness of less than 30 ppm. calculated as; calcium to a recycling mother liquor containing pri marily water and dissolved sodium carbonate. and sodium bicarbonate to provide a dissolver feed solution having,

less than 30 ppm. calcium hardness, dissolving said crude trona in said recycling mother liquor to provide a dissolver discharge solution having less than 40 ppm. calcium hardness, clarifying said solution to remove insolubles therefrom and provide a crystallizer feed liquor, maintain-: ing a calcium concentration in said crystallizer feed liquor of less than 40 ppm., adding an anionic-active surfactant taining calcium therein, which crystals are improved size, clarity, dewaterability and settling rate, which com-.

prises softening the natural hard water of the region, con-.

taining an average total hardness of about 85 to 120 ppm. calculated as calcium to a calcium hardness of less than 20 ppm. to provide make-up water for said process, adding said make-up water softened to a calcium hardness of less than 20 ppm. calculated as calcium to a recycling mother liquor containing primarily Water and dissolved sodium carbonateand sodium bicarbonate in such amount as .to provide a dissolver feed solution having less than 25 ppm. calcium hardness, dissolving said crude trona in said recycling mother liquor =to provide a dissolver discharge solution having less than 30 ppm. calcium hardness, clarifying said solution to remove insolubles therefrom and provide a clarified crystallizer feed liquor, settling and removing said insolubles from said crystallizer feed liquor and slurrying the insolubleswith hard natural water ;to dissolve further sodium values therefrom and soften said hard natural Water to a calcium hardnessof less than 20 ppm., maintaining a calcium concentration in the clarified crystallizer feed liquor of-less than 30 ppm., adding an anionic-active surfactant to said clarified solution in the amount of about to about 100 ppm., crystallizing, precipitating and removing sodium sesquicarbonate crystals from said solution, recycling the mother liquor from the crystal removal step to dissolve more trona, separating the softened water from the trona insolubles and adding said softened water as make-up water to the recycling mother liquor.

3. 'A process for preparing crystals of sodium sesquicarbonate from Wyoming trona whichcrystals are improved in size, dewaterability and settling rate, which comprises softening the naturalhard water of the region containing an average total hardness of 85 to 120. ppm. calculated as calcium to a calcium hardness of less than 30 ppm. to provide make-up water for said process, forming a plant dissolver solution byadding said softened make-up water to a recycling mother liquor containing primarily water and dissolved sodium carbonate and sodium bicar: bonate to provide an aqueous solution containing less than 30 ppm. of calcium therein, dissolving crude trona in said solution to form a crystallizer solution, maintaining. a calcium concentration of less than 40 ppm. in said crystallizer feed solution, crystallizing and precipitating so-: dium sesquicarbonate from said solution by effecting the crystallization and precipitation of said salt from said solution in the presence of about 5 to about 100 ppm. of an anionic-active surfactant agent selected from the group consisting of (1) alkyl benzene s-ulfonatescontaining at least 8 alkyl carbon atoms, (2) alkyl naphthalene sulfonates containing at least 4 alkyl carbon atoms, (3) primary alkyl alcohol sulfonates containing at least carbon atoms, (4) N-substituted taurines of'the formula R7R".NCH CH SO M where R is a hydrocarbon radical,

12 E R" 'isthe acyl radical, of a higherfatty. acid and M is an alkali metaLseparatingthe sodium sesquicarbonate crystals from said solution and recycling themother liquor to.

dissolve more, trona..

4.: A'processfor preparing crystals of sodium sesqui-5 carbonate from crude Green River, Wyoming trona, which comprises softening the naturalhard water. of the region containing an average total hardness of about to ppm. .calcula'tedas calcium to a calcium hardness of lessthan 20 ppm. to provide make-up'water; for said process, adding said make-up water softened to a calcium hardness of'less than 20; ppm. as calcium to a recycling mother liquor containing primarily water :and dissolved sodiumcarbonate and sodium vbicarbonate to provide a dissolverfeed-solution havingless than 25 ppm. calcium. hardness,

dissolving said crude trona insaid recycling mother liquor to provide a dissolver discharge; solution having less than 30 ppm. calcium hardness, clarifying said solution .to remove insolubles therefrom and providega'crystallizer feed solution, settling and removing said insolubles and slurrying the'insolubles withhard natural water to dissolve further sodium values-therefrom and soften said hard natural water. to a calciumhardness; of less than 20 ppm. mains tainingacalciu'm concentration of less than 30 ppm. in

the clarified crystallizer. feed solution, adding an anionic active surfactant crystallization modifierto. said solution, crystallizing, precipitating and removing sodium sesquicarbonate crystals of low calcium content from said solution, recyclingthe .mother liquor fromthe crystallization step to..dissolve .more.trona,-separating the softened waterfrom the trona insolubles and adding said softened water as make-up water to the recycling mother liquor.

5.:The process of claim4 wherein the'salt being cryse tallized is sodium sesquicarbonate and thesurfactantis' present in the: aqueous ;.solution prior to crystallization in a concentration of from about 510. a-bout20 ppm. active .material and the calcium content ofthe crystallizing solution is less than 30 ppm.

6. The process of claim .4'wherein the surfactant is an 7 alkyl benzene sulfonate containing at least 8 carbon atoms.

References Cited :by the Examiner UNITED STATES PATENTS l 1,766,705 6/ 1930 Dehnel 23-64 2,155,477 4/1939 Drujon 23-64 2,346,140 4/1944 Pike .2363 X 2,591,704 4/1952 King 23-300 X 2,595,238 5/1952 Frejacques 23--300 2,607,660 8/1952 Robertson 23964 2,639,217 5/1953 Pike z 23..' 63 I 2,670,269 2/1954 Rahn 23-63 2,704,239 3/1955 Pike 2. 23-63 f 2,720,446" 10/1955 Whetstone 23-300 X 2,780,520 2/ 19.57 Pike 23"63 2,954,282. 9/ 1960 Bauer et al; 23 300 OTHER REFERENCES 1 NORMAN YUDKOFF, Primary Examiner.

ANTHONY; SCIAMANA, 'MAURICE' BRINDISI,

Examiners. 

1. A PROCESS FOR PREPARING CRYSTALS OF SODIUM SESQUICARBONATE FROM CRUDE GREEN RIVER, WYOMING TRONA, WHICH CRYSTALS ARE IMPROVED IN SIZE, CLARITY, DEWATERABILITY AND SETTLING RATE, WHICH COMPRISES SOFTENING THE NATURAL HARD WATER OF THE REGION, CONTAINING AN AVERAGE TOTAL HARDNESS OF ABOUT 85 TO 120 PPM. CALCULATED AS CALCIUM TO A CALCIUM HARDNESS OF LESS THAN 300 PPM. TO PROVIDE MAKE-UP WATER FOR SAID PROCESS, ADDING SAID MAKE-UP WATER SOFTENED TO A CALCIUM HARDNESS OF LESS THAN 30 PPM. CALCULATED AS CALCIUM TO A RECYCLING MOTHER LIQUOR CONTAINING PRIMARILY WATER AND DISSOLVED SODIUM CARBONATE AND SODIUM BICARBONATE TO PROVIDE A DISSOLVER FEED SOLUTION HAVING LESS THAN 30 PPM. CALCIUM HARDNESS, DISSOLVING, SAID CRUDE TRONA IN SAID RECYCLING MOTHER LIQUOR TO PROVIDE A DISSOLVER DISCHARGE SOLUTION HAVING LESS THAN 40 PPM. CALCIUM HARDNESS, CLARIFYING SAID SOLUTION TO REMOVE INSOLUBLES THEREFROM AND PROVIDE A CRYSTALLIZER FEED LIQUOR, MAINTAINING A CALCIUM CONCENTRATION IN SAID CRYSTALLIZER FEED LIQUOR OF LESS THAN 40 PPM., ADDING AN ANIONIC-ACTIVE SURFACTANT CRYSTALLIZATION MODIFIER TO SAID SOLUTION IN THE AMOUNT OF ABOUT 5 TO ABOUT 100 PPM., PRECIPITATING AND REMOVING SODIUM SESQUICARBONATE CRYSTALS FROM SAID SOLUTION AND RECYCLING THE MOTHER LIQUOR FROM THE CRYSTAL REMOVAL STEP TO DISSOLVE MORE TRONA. 